I think the reason for 404 error is the load time.. Your host is not good and the webpage takes time to load and because of that the meta checkers bot cannot crawl the page which in return gives a 404 error...

So in essence, what is appearing is a 404 error page on the browser - its just showing the content for the page. You need to take a look at your WP plugins and server config - perhaps disable and cache management and minify plugins and see if that helps.

I use Chrome - but for easy troubleshooting perhaps you should go with Firefox and the live HTTP headers plugin.

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